1. Field of the Invention
The present invention relates to methods of treating subterranean zones in formations penetrated by well bores with viscosified and thixotropic aqueous treating fluids.
2. Description of the Prior Art
Viscous treating fluids are used in a variety of operations and treatments in oil and gas wells. Such operations and treatments include formation permeability conformance control treatments, treatments to reduce undesirable well bore fluid inflows and outflows and others.
An example of a formation permeability conformance control treatment involves enhanced recovery techniques such as water flooding. In water flooding, an aqueous displacement fluid is injected under pressure into an oil containing subterranean formation by way of one or more injection wells. The flow of the aqueous fluid through the formation displaces oil contained therein and drives it to one or more producing wells. However, the aqueous displacement fluid often flows through the most permeable zones in the subterranean formation whereby less permeable zones containing oil are bypassed.
Heretofore, enhanced recovery problems have been corrected by reducing the permeability of the subterranean flow paths having high permeability and low oil content. As a result, the subsequently injected aqueous displacement fluid is forced through flow paths of low permeability and high oil content. The techniques utilized to accomplish the high flow path permeability reduction have included injecting aqueous solutions of polymers into the high permeability flow paths whereby the polymers are gelled and cross-linked therein.
An example of a treatment to reduce undesirable well bore fluid inflows and outflows involves the production of oil from oil-bearing formations containing highly permeable water channels communicating the well with water zones whereby water flows into the well. Heretofore, polymerizable monomers, cross-linking agents and delayed polymerization initiators have been introduced into the water channels wherein the monomers are polymerized and cross-linked so that the water channels are plugged.
For viscous well treating fluids to function properly, the fluids must remain viscous until the desired well operation or treatment has been completed. Often, however, prolonged exposure of a viscous well treating fluid to high temperatures encountered in a well causes the fluid to prematurely degrade and lose its viscosity. Many viscous treating fluids utilized heretofore lose significant viscosity in a few hours at the temperatures experienced down hole, usually at temperatures above about 300xc2x0 F. Preferably, when required, a viscous well treating fluid should retain viscosity for up to 12 hours at temperatures from about ambient up to and including 400xc2x0 F.
Thixotropy imparting agents have heretofore been utilized in highly viscous well treating fluids. Thixotropy imparting additives give the highly viscous treating fluids the ability to lose viscosity when being sheared such as by pumping and to return to the highly viscous gelled form when at rest. A highly viscous treating fluid having thixotropic properties allows the fluid to be readily pumped to and placed in a subterranean zone and then regain its highly viscous properties after placement. While various thixotropy imparting additives have heretofore been known and used in the art, such additives have been substantially ineffective in subterranean zones having temperatures greater than about 250xc2x0 F.
Thus, there are needs for improved methods of treating subterranean zones penetrated by well bores with high viscosity treating fluids that have thixotropic properties and retain their high viscosity at subterranean zone temperatures from about 250xc2x0 F. up to and including 400xc2x0 F.
The present invention provides improved methods of treating subterranean zones penetrated by well bores, the subterranean zones having static temperatures from about ambient up to and including 400xc2x0 F. The methods are basically comprised of introducing into a subterranean zone by way of a well bore, an aqueous treating fluid comprised of an aqueous liquid containing one or more inorganic monovalent salts, multivalent salts or both in an amount no greater than about 5% by weight of the aqueous liquid and one or more synthetic hectorite clay gelling and thixotropy imparting agents selected from the group of a synthetic hectorite clay having the composition by weight on a dry basis of 59.5% SiO2, 27.5% MgO, 0.8% Li2O and 2.8% Na2O, a synthetic hectorite clay having the composition by weight on a dry basis of 54.5% SiO2, 26.0% MgO, 0.8% Li2O, 5.6% Na2O and 4.1% P2O5 and a synthetic hectorite clay having the composition by weight on a dry basis of 54.5% SiO2, 26.0% MGO, 0.8% LiO2, 5.6% Na2O, 4.1% P2O5 and also containing inorganic polyphosphate peptizers.
When one or more of the gelling and thixotropy imparting agents of this invention are added to an aqueous treating fluid, the viscosity of the fluid is increased and the fluid has thixotropic properties whereby the viscosity of the aqueous treating fluid is reduced during pumping and placement in a subterranean zone, but regains a high viscosity when static. In order to stabilize the viscosity during pumping a separate viscosity stabilizing gelling agent can be included in the treating fluid. Examples of commonly used gelling agents include galactomannans, modified or derivatized galactomannans and cellulose derivatives. Cross-linking agents, breakers and other additives can also be included in the viscous well treating fluids useful in accordance with this invention.
Subterranean formation permeability conformance control treatments are also provided by the present invention. Such methods are basically comprised of altering the permeability of the subterranean formation by contacting the subterranean formation with an aqueous treating liquid containing one or more inorganic monovalent salts, multivalent salts or both in an amount no greater than about 5% by weight of the aqueous liquid and one or more synthetic hectorite clay gelling and thixotropy imparting agents selected from the group set forth above. The aqueous treating liquid can also include separate gelling agents of the types mentioned above, cross-linking agents and other additives. A particularly suitable separate gelling agent which can be used is a water dispersible hydrophilic organic polymer having a molecular weight greater than 100,000 and containing carboxyl functionality and a zirconium cross-linking composition. Another particularly suitable separate gelling agent which can be used is a chelated organic gelling agent comprised of a metal ion chelated water soluble polyalkaline imine and a water soluble polymer capable of being cross-linked by the chelated organic gelling agent. The metal ion chelated water soluble polyalkaline imine can be selected from polyethylene imine or polypropylene imine and the water soluble polymer capable of being cross-linked by the chelated organic gelling agent is a terpolymer of an ethylenically unsaturated polar monomer, an ethylenically unsaturated ester and a monomer selected from the group consisting of 2-acrylamido-2-methylpropane sulfonic acid (AMPS(copyright)) or N-vinylpyrrolidone or the water soluble polymer is a tetrapolymer of an ethylenically unsaturated polar monomer, an ethylenically unsaturated ester, AMPS(copyright) and N-vinylpyrrolidone.
Treatments to reduce undesirable well bore fluid inflows and outflows from a subterranean zone are also provided by this invention. Such methods are basically comprised of the following steps. An aqueous treating fluid is placed in the subterranean zone which contains one or more inorganic monovalent salts, multivalent salts or both in an amount no greater than about 5% by weight of the aqueous treating fluid and one or more synthetic hectorite clay gelling and thixotropy imparting agents selected from the group set forth above. The aqueous treating fluid also includes a separate gelling agent of the type mentioned above, cross-linking agents and other additives. After the subterranean zone is contacted, the aqueous treating fluid is allowed to remain in the subterranean zone at rest for a time within which the gelling agents therein form a solidified sealing mass in the subterranean zone. A particularly suitable separate gelling agent which can be used is a polymerizable water soluble vinyl monomer and a multifunctional vinyl monomer cross-linker together with a water soluble azo initiator.
It is, therefore, a general object of the present invention to provide improved methods of treating subterranean zones in formations penetrated by well bores.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows when taken in conjunction with the accompanying drawing.